There was a Japanese isekai with a guy who got a kar98 to use as his weapon and he also has a bunny girl as a companion. I believe it has a manga as well
That's the one. Thanks https://www.novelupdates.com/series/magan-to-dangan-o-tsukatte-isekai-o-buchinuku/
The guy above found it. https://www.novelupdates.com/series/magan-to-dangan-o-tsukatte-isekai-o-buchinuku/